Prior art piezoresistive pressure dies are based on plain or bossed diaphragms. Strain gages are placed on the diaphragms to sense the strains created by the pressure. The strains depend on diaphragm geometry. In order to produce adequate strains in low pressure range sensors, thin diaphragms must be used. Thin diaphragms exhibit a number of problems: Ballooning; Mid-plane stretching under applied load; Buckling; Sensitivity to in plane stresses by thermal expansion; Bi-metallic thermal effects; Relatively large boss required to implement the required stress concentration.
This causes performance problems: High thermal zero shift; Anomalous thermal span shifts (lower or higher); Nonlinearity; Zero instability; Gravity/acceleration sensitivity. All low pressure silicon piezoresistive sensors currently offer compromised performance: Limited to higher than desirable range; Down-rated; Have higher nonlinearity, zero drift and long term drift as compared to higher ranges specifications; Suffer from lower yields; Are Larger and more expensive; Suffer from excessive “g” sensitivity due to the large mass of the boss.
Past solutions to this problem were explored: Replacing the flat diaphragm with a single or double bossed diaphragm; Peripherally thinned bossed diaphragms. These innovations improved low pressure range performance, but fall short of needed specifications.
The crux of the problem is that current technology, which employs diaphragms to act as both the force collector and the sensing flexure, is too rigidly constrained to perform both tasks optimally. It is, therefore, the object of the present invention to provide an integral diaphragm-flexure structure, implemented in a single layer structure and a two layer structure, which incorporates the following features: Provides sufficient independent variables to optimize sensitivity and linearity; Removes piezoresistive strain gages from the thin diaphragm; Places them on a thicker beam (integral of linked); Employs no boss or a very small linking boss.